The present invention relates to a method of producing an engine wall structure that comprises an inner wall, to which hot gas is admitted during engine operation, an outer wall, which is colder than the inner wall during engine operation, and at least two webs that connect the inner wall with the outer wall and delimit a cooling duct between said walls.
During engine operation, any cooling medium may flow through the ducts. However, in particular, the invention relates to engine wall structures and a process for manufacturing engine wall structures in which there is a plurality of such webs dividing the space between the walls into a plurality of ducts, in particular for cooling the firing chamber walls and the thrust nozzle walls of rocket engines driven with hydrogen as a fuel or hydrocarbon, i.e. kerosene, wherein the fuel is introduced in the cold state into the wall structure, is delivered through the cooling ducts while absorbing heat via the inner wall, and is subsequently used to generate the thrust. Heat is transferred from the hot gases to the inner wall, further on to the fuel, from the fuel to the outer wall, and, finally, from the outer wall to any medium surrounding it. Heat is also transported away by the coolant media as the coolant temperature increases by the cooling. The hot gases may comprise a flame generated by a combustion of gases and/or fuel.
Accordingly, the engine wall structure is preferably a thrust nozzle wall, preferably of a rocket engine. The inner wall of such a nozzle has, primarily, a heat exchanging function, while the outer wall, primarily, has a load carrying function, the thickness of the inner wall being substantially less than the one of the outer wall.
According to prior art, thrust nozzle walls of rocket engines are constructed as a sandwich construction comprising an inner wall and an outer wall connected by webs that run in the lengthwise direction of the nozzle wall and delimit a plurality of ducts between the walls. The ducts are used as cooling ducts through which a cooling medium is permitted to flow. The cooling medium may comprise the engine fuel which is routed back to the combustion chamber after cooling, whereby the cooling is called regenerative cooling. On the other hand, the cooling medium might be a medium not primarily used for further purposes than cooling, whereby the cooling is called dump cooling. Also in this case the medium may comprise fuel, however not used for subsequent combustion.
Normally, the inner wall of the thrust nozzle mainly acts as a heat exchanger between the cooling medium and the hot gases on the inside of the inner wall, while the webs and the outer wall primarily has a load carrying function. Preferably the inner wall should have a relatively small thickness, for example in the range of 0.15-1.5 mm. Also, a variation as small as possible of the inner wall thickness is required, as variation in the thickness of the inner wall will result in varying stresses and strains of the inner wall and large functional variations of the inner wall temperature during operation.
According to prior art, as for example disclosed in U.S. Pat. No. 6,640,538, the inner wall, webs and ducts of a combustion chamber wall are produced by milling or electro eroding an inner sheet that will form the inner wall, such that open grooves are formed on the side thereof directed towards the outside or outer wall. Subsequently, a sheet forming the outer wall is applied onto the webs of the inner wall, thereby sealing and defining the ducts. Similar methods have been suggested for the production of thrust nozzle walls, however without any suggested use of electro-erosive processes for accomplishing the ducts.
However, connecting the outer wall to the inner wall may be rather cumbersome and costly. Moreover, especially if milling is used as the process for generating the ducts, or grooves, it will be difficult to achieve a required tolerance as to the thickness of the inner wall.
It is desirable to present a method of producing an engine wall structure as initially defined that is cost efficient in relation to methods of prior art.
It is also desirable to present a method of producing an engine wall structure as initially defined, by which it will be possible to achieve a very high tolerance as to the thickness of the inner wall. The method should also be well adapted for the purpose of producing such engine wall structures with ducts of rather complex cross-sectional shape, or ducts that, for example, get wider in the longitudinal direction thereof.
According to a method aspect of the present invention, the engine wall structure is produced by wire-electro discharge machining the duct out of a solid sheet forming the entire engine wall structure including the inner wall, the outer wall and the webs. Wire-electro discharge machining will hereinafter be referred to as wire-EDM
According to an aspect, the wire used for the wire-EDM operation is introduced into the solid sheet from the side thereof forming the outer wall. Thereby, slits caused by the wire that need to be sealed are avoided at the inner wall surface.
According to a further aspect, after having cut out the duct, the wire used for the wire-EDM is guided out of the solid sheet via the same slit that was generated upon introduction of the wire into the sheet. Thereby, only one slit per produced duct will require sealing after the wire-EDM of the duct in question.
According to another aspect, the engine wall structure is to be provided with at least two adjacent ducts, and the wire used for the wire-EDM is introduced into the solid sheet in a region between said ducts up to a diverging junction from which it is guided to and used for cutting out a first of said ducts, subsequently guided back to said diverging junction, then guided to and used for cutting out the second one of said ducts, and finally guided out of the solid sheet via the diverging junction and a slit that was generated upon the introduction of the wire into the solid sheet. Thereby, only one slit will require sealing after the wire-EDM of the pair of ducts in question. Said diverging junction is sealed for preventing communication between the two adjacent ducts. Preferably, the diverging junction is sealed by a metal fusion process.
According to an alternative aspect, the wire used for the wire-EDM operation is introduced into the solid sheet along a first path and guided out of the sheet along a second path, said first and second paths ending in the created duct, thereby leaving a generally wedge-shaped body between the first and second paths, said body-then being displaced in a direction towards the duct in order to fit in as a sealing means for sealing the slits generated in the solid sheet along the first and second paths, and finally connected to the wall in which it is fitted.
In order to perfectly counteract any cooling medium leakage due to a communication between the duct and the environment via the interface between the wedge-shaped body and the adjacent wall material, said interface is further sealed by means of a metal fusion process, preferably welding. That same metal fusion process could also be used for connecting the wedge-shaped body to the surrounding wall material.
Generally, according to the invention, the slit or slits generated in the solid sheet upon introduction or removal of the wire used for the wire-EDM should be sealed in order to inhibit any communication between the duct or ducts and the surrounding environment via said slit or slits, thereby preventing unwanted leakage of the cooling medium from the duct during operation.
Preferably, the slit or slits are sealed by a metal fusion process.